HUMAN GTP CYCLOHYDROLASE-I - ONLY ONE OUT OF 3 CDNA ISOFORMS GIVES RISE TO THE ACTIVE ENZYME

GTP cyclohydrolase I catalyses the first and rate-limiting step of tet rahydrobiopterin biosynthesis. Its expression is regulated by interfer on-gamma or kit ligand in a tissue-specific manner. Three different cD NA forms have been reported for human GTP cyclohydrolase I [Togari, Ic hinose, Matsumoto, Fujita and Nagatsu (1992) Biochem. Biophys. Res. Co mmun. 187, 359-365]. We have isolated, from a human liver cDNA library , two clones which contained inserts identical with two of the cDNAs r eported by Togari et al. (1992). The three open reading frames corresp onding to all reported cDNA sequences were expressed in Escherichia co li. Only the recombinant protein corresponding to the longest reading frame catalysed the conversion of GTP into dihydroneopterin triphospha te. The proteins corresponding to the shorter reading frames failed to catalyse not only the generation of dihydroneopterin triphosphate but also the release of formate from GTP, an intermediate step of the rea ction. Recombinant human GTP cyclohydrolase I showed sigmoidal substra te kinetics and maximum activity at 60 degrees C. These findings are w ell in line with the published properties of the enzyme isolated from rat liver. The data indicate that cytokine-mediated induction of GTP c yclohydrolase I is not due to the expression of enzyme isoforms.